Harvey J. Stiegler

Hebbian Learning in Spiking Neural Networks With Nanocrystalline Silicon TFTs and Memristive Synapses

Characteristics similar to biological neurons are demonstrated in SPICE simulations of spiking neuron circuits comprised of submicron nanocrystalline silicon (nc-Si) thin-film transistors (TFTs). Utilizing these neuron circuits and corresponding device models, the properties of a two-neuron network are explored. The synaptic connection consists of a single nc-Si TFT and a memristor whose conductance determines the synaptic weight. During correlated spiking of the pre- and postsynaptic neurons, the strength of the synaptic connection increases. Conversely, it is diminished when the spiking is uncorrelated. This synaptic plasticity and Hebbian learning are essential for performing useful computation and adaptation in large-scale artificial neural networks. The importance of the result is augmented by the fact that these properties are demonstrated using models based on measured data from devices with potential for 3-D integration into a nanoscale architecture with extremely high device density.

Neural Learning Circuits Utilizing Nano-Crystalline Silicon Transistors and Memristors

Properties of neural circuits are demonstrated via SPICE simulations and their applications are discussed. The neuron and synapse subcircuits include ambipolar nano-crystalline silicon transistor and memristor device models based on measured data. Neuron circuit characteristics and the Hebbian synaptic learning rule are shown to be similar to biology. Changes in the average firing rate learning rule depending on various circuit parameters are also presented. The subcircuits are then connected into larger neural networks that demonstrate fundamental properties including associative learning and pulse coincidence detection. Learned extraction of a fundamental frequency component from noisy inputs is demonstrated. It is then shown that if the fundamental sinusoid of one neuron input is out of phase with the rest, its synaptic connection changes differently than the others. Such behavior indicates that the system can learn to detect which signals are important in the general population, and that there is a spike-timing-dependent component of the learning mechanism. Finally, future circuit design and considerations are discussed, including requirements for the memristive device.

Control and stability of self-assembled monolayers under biosensing conditions

The ability to stabilize and control the attachment of cells on the surfaces of a variety of inorganic materials is important for the development of biomedical devices and sensors. An important intermediate step is the functionalization of semiconducting surfaces with a self-assembled monolayer (SAM) with an appropriate surface termination to interact with proteins. The stability of such SAMs is critical to withstanding subsequent processing and measurement conditions (e.g. long exposure to a buffer solution) to avoid artifacts resulting from such deterioration during electrical measurements. This work highlights the importance of surface cleaning and SAM chain length by comparing two commonly used short alkyl chains, aminopropyltriethoxysilane (APTES) or 3-(trimethoxysilyl)propyl aldehyde (C4-ald) molecules, with their long-chain counterparts, amino-undecilenyltriethoxysilane (AUTES) and 11-(triethoxysilyl)undecanal (C11-ald). Using IR spectroscopy, spectroscopic ellipsometry, and electrical measurements, a cleaning method is developed, based on a short room temperature (RT) SC-1 treatment, to remove photoresist without degrading device performance as is the case with currently used oxygen plasma methods. The spectroscopic and electrical measurements also show that short-chain SAMs, typically used for pH- or bio-sensing, do not have the stability suitable for biosensor environments. In contrast, long-chain SAMs display much higher stability and can be reproducibly grafted. These findings are the basis for a reliable preparation and robust operation of biosensors.

14MHz organic diodes fabricated using photolithographic processes

Organic semiconductor-based Schottky diodes operating at 14MHz, fabricated using conventional photolithographic and etching processes, have been demonstrated. Copper phthalocyanine is the semiconductor, with gold and aluminum as the Ohmic and Schottky contacts, respectively. The organic diode based rectifier circuit generated a dc output voltage of approximately 2V at 14MHz, using an input ac signal with a zero-to-peak voltage amplitude of 5V. These devices showed little degradation under continuous ac voltage stress when operated in vacuum.

Polarization behavior of poly"vinylidene fluoride-trifluoroethylene… copolymer ferroelectric thin film capacitors for nonvolatile memory application in flexible electronics

The time domain and electric field dependence of the polarization switching kinetics of poly(vinylidene fluoride-trifluoroethylene) copolymer based thin film metal-ferroelectric-metal capacitors have been characterized. At room temperature, the time required for complete switching polarization decreases from >1 s to <50 μs as the voltage is increased from 6 to 12 V, while low nonswitching polarization is maintained. In the time domain, the ferroelectric switching polarization reversal behavior for devices biased above the coercive field follows the nucleation-limited-switching model. The exponential relationship between switching time and applied electric field indicates nucleation dominated switching kinetics. Switching behavior as a function of temperature was also characterized from −60 to 100 °C in the voltage range of 6–12 V. Higher temperatures induce larger dc conductance leakage at low frequencies and increases nonswitching polarization for all the voltages studied. It is demonstrated that for cer...

One-step selective chemistry for silicon-on-insulator sensor geometries.

A one-step functionalization process has been developed for oxide-free channels of field effect transistor structures, enabling a self-selective grafting of receptor molecules on the device active area, while protecting the nonactive part from nonspecific attachment of target molecules. Characterization of the self-organized chemical process is performed on both Si(100) and SiO(2) surfaces by infrared and X-ray photoelectron spectroscopy, atomic force microscopy, and electrical measurements. This selective functionalization leads to structures with better chemical stability, reproducibility, and reliability than current SiO(2)-based devices using silane molecules.

Low-Temperature Hybrid CMOS Circuits Based on Chalcogenides and Organic TFTs

In this letter, we demonstrate a fully integrated approach to fabricate cadmium sulfide (CdS)-pentacene complementary metal-oxide-semiconductor (CMOS) digital circuits compatible with flexible electronics. Low-cost and low-temperature chemical bath deposition is used to deposit CdS at 70°C with mobility values >; 10 cm2/V·s and threshold voltages around 5 V for fully integrated devices. p-MOS thin-film transistors were fabricated using thermally evaporated pentacene as semiconductor with mobility and threshold voltages in the range of 3×10-2 cm2/V·s and -3 V, respectively. The CMOS integration approach includes six mask levels with a maximum processing temperature of 100°C.

Submicron Ambipolar Nanocrystalline Silicon Thin-Film Transistors and Inverters

Nanocrystalline silicon (nc-Si) thin-film transistors (TFTs) fabricated at a maximum processing temperature of 250 °C operate with high field-effect mobility compared with amorphous-silicon TFTs. By reducing the oxygen content in the channel layer, ambipolar behavior can be obtained. Two levels of electron-beam lithography are employed to fabricate nc-Si TFTs with nanoscale dimensions that operate without significant short-channel effects for gate lengths down to 200 nm. The TFTs have current-voltage (I- V) characteristics with on-off ratio >; 105 at ±1 V drain voltage and low threshold voltage shift. Simulation Program with Integrated Circuit Emphasis (SPICE) software is used to model the TFTs, and it is validated by performing the fit to devices of different dimensions. An inverter constituent of nc-Si TFTs offers high voltage gain (10-12) and frequency response better than 2 MHz. The crowbar current associated with the inverter can be minimized by using an optimized geometry ratio based on the leakage currents of the TFTs. An amplifier circuit is also demonstrated, offering an ac gain in the frequency range of 100 Hz-10 kHz. SPICE simulations of the inverter and amplifier show close agreement with measured data. The fabricated devices are well suited for use in high-density architectures.

Comparison of Methods to Bias Fully Depleted SOI-Based MOSFET Nanoribbon pH Sensors

The potential and electric field boundary conditions for the Gouy-Chapman model of the electrolyte diffuse layer are used to properly couple the potentials in the silicon-on-insulator-based metal-oxide-semiconductor field-effect transistor to the electrolyte. This analysis is possible because the active silicon channel is fully depleted. Both the subthreshold and linear regimes are simulated. An operation with electrolyte floating and bias applied to the substrate is compared with the other methods of biasing the sensor.

Effect of mobile ions on ultrathin silicon-on-insulator-based sensors

The presence of mobile Na+ and K+ ions in biological solutions often lead to instabilities in metal-oxide-semiconductor devices and is therefore an important consideration in developing sensor technologies. Permanent hysteresis is observed on silicon-on-insulator field-effect-transistors based sensors after exposure to Na+-based buffer solutions but not after exposure to K+-based solutions. This behavior is attributed to the difference in mobilities of the ions in silicon dioxide. Mobile charge measurements confirm that ions can be transferred from the solution into the oxide. Self-assembled monolayers are shown to provide protection against ion diffusion, preventing permanent hysteresis of the sensors after exposure to solutions.